The present invention generally pertains to subscriber communications systems and is particularly directed to an improved subscriber unit for wireless communication with a base station in a wireless digital subscriber communication system.
A typical subscriber unit is described in U.S. patent application Ser. No. 06/893,916 filed Aug. 7, 1986 by David N. Critchlow et al. Now U.S. Pat. No. 4,825,448. A base station used with such a subscriber unit in a wireless digital subscriber communication system is described in U.S. Pat. No. 4,777,633 to Thomas E. Fletcher, Wendeline R. Avis, Gregory T. Saffee and Karle J. Johnson. The subscriber unit described in U.S. Pat. No. 4,825,448 includes means for transcoding a digital voice input signal to provide digital input symbols; means for FIR filtering the digital input symbols; means for deriving an analog intermediate frequency input signal from the filtered input symbols; means for combining the intermediate frequency input signal with an RF carrier for radio transmission to the base station; means for demodulating an output signal received from the base station to provide digital output symbols; and means for synthesizing a digital voice output signal from the digital output symbols. The subscriber unit includes a baseband processor chip and a modem processor chip. Both are TMS32020 digital signal processors. The baseband processor chips perform the transcoding of the digital voice input signal, the synthesis of the digital output symbols, and various baseband control functions; and the modem processor chip performs the FIR filtering of the digital input symbols, and the demodulation of the output signal received from the base station. The modem processor chip generally acts as the master for the system.
A subscriber unit has a processor. The processor provides an output phase signal corresponding to a selected output digital frequency. A tuning register buffers the phase signal. A lookup table has two sets of predefined stored values pertaining to the amplitude of a signal for a single quadrant. The predefined stored values comprise coarse angle approximations and fine angle approximations. A sine and cosine generator receives the phase signal and generates sine and cosine waveforms utilizing amplitude values obtained from the lookup table. The phase signal includes phase data and specifies the quadrant and the algebraic sign of the phase data. The sine and cosine generator accessing the lookup table differently depending upon the quadrant and sine of the phase data, such that the lookup table provides an amplitude value from the sets of predefined stored values based on the phase data. A modulator combines the sine and cosine waveforms to produce the selected output digital frequency and modulates digital frequency.